Band Structure Topology and Spin Transport in Magnon Systems

As the spin excitation quanta in magnetic materials, the magnon is at the heart of the spintronics research because it plays a key role in magnetic dynamics, energy and spin transport, and even determining the ground state of magnetic systems. In this thesis, we will study the band-structure topology and transport properties of magnons in both collinear and noncollinear magnets. Inspired by the great success of topological insulators, exploring magnon topology can unveil the topological nature of bosonic particles and widen the zoo of topological materials. We propose a three-dimensional magnon topological insulator model protected by sublattice chiral symmetries, which realizes a surface Dirac cone in a magnonic system. On the other hand, magnons can facilitate angular momentum transport with low dissipation due to the absence of Joule heating. We explore the spin Nernst effect, a transverse spin current driven by a temperature gradient, in noncollinear magnetic systems by developing a new linear response theory. The theory will be applied to frustrated noncollinear antiferromagnets, antiferromagnetic skyrmion crystals, and an antiferromagnetic magnon topological insulator model. In particular, the antiferromagnetic magnon topological insulator model is featured by unconventional Landau levels and can be regarded as a magnon version of the quantum spin Hall effect. In addition to the magnon-mediated spin transport, magnons are also able to accumulate nonequilibrium net spin density in a sample under the driving of a temperature gradient. The latter effect is a magnon version of the Edelstein effect and can be also analyzed by the aforementioned linear response theory. Such an effect can be ideally realized in 2D and 3D noncollinear antiferromagnets that have a compensating ground state